Heat exchanger elements formed from primary structure are considered to be more thermally efficient than fabricated heat exchanger elements which constitute "secondary structure" and which may include a number of boundaries constituted by welds or joins. Thus not only is there a possibility of fluid leaks occurring in relation to these boundaries but also such boundaries provide zones of resistance to conduction of heat to heat transmission surfaces of the heat exchanger element.
Heat exchanger elements formed from primary structure are well known and reference may be made for example to U.S. Pat. Nos. 3,202,212; 4,565,244; 3,743,252; 4,352,008; 3,566,959; 4,567,074; 3,137,785 and 3,467,180 which all show that formation of one piece extrusions as heat exchanger elements are not new.
Reference, however, may be made to GB Patent 2,142,129 which describes a heat exchanger element or core which was incorporated in a radiator for use in a central heating system. The heat exchanger element was in the form of a rectangular elongate hollow body which is provided on each of the opposite sides with a plurality of spaced heat radiating fins. However, in this reference, there was no particular use described of the heat exchanger element per se and reference was only made to a radiator incorporating the heat exchanger element. The radiator included a cover plate which extended across the free ends of each set of fins so as to provide a plurality of open ended channels through which air could flow which air was heated by a transfer of heat from a hot fluid which could flow through the rectangular hollow body. This reference also describes an embodiment using a multiplicity of rectangular hollow sections. In such embodiment, to form the radiator, each hollow section had to be welded to each other and the terminal hollow sections provided with cover plates.
A heat exchanger comprising a plurality of modules each having a plurality of channels spaced from each other and located in a central body part and also a plurality of fins extending outwardly on each side of the central body part is also described in U.S. Pat. No. 4,401,155.
Of particular interest, in relation to the present invention, is the chemical milling of aluminium extrusions of the type described in GB Patent 2,142,129. Thus, the invention is concerned particularly with the chemical milling of aluminium heat exchanger elements having an elongate hollow body bounded by a peripheral wall and also having a plurality of fins extending outwardly from the peripheral wall. However, it will be appreciated by the skilled addressee that the chemical milling apparatus and method may also be applied to other heat exchanger elements having different shapes as hereinafter described and which, for example, are described in the prior art aluminium extrusions referred to above.
In this regard, it is useful in relation to heat exchanger elements, to be able to carry out a chemical milling or etching process so as to reduce the weight of metal which therefore provides a lighter heat exchanger element.
Reference may be made to "The Chem-Mill Design Manual" prepared by the Chem-Mill and Coatings Division of Turco Industries Inc in the United States which provides basic information on chemical milling processes and describes that the essential requirement of conventional chemical milling processes is to provide the fabrication of light weight metal parts of high strength which before the advent of chemical milling were either prohibitively expensive or impossible to manufacture using conventional mechanical processes.
In the above publication, chemical milling is defined as a process used to shape metals to an exacting tolerance by the chemical removal of metal or deep etching of parts rather than by the use of conventional mechanical, milling or machining operations.
The advantage of chemically milling a metal article, such as a heat exchanger element, is to remove metal chemically so that the heat exchanger element may be shaped to a desired shape or tolerance. The amount of metal removed or the depth of etch is controlled by the time of immersion in an etching solution. The location of unetched or unmilled areas on the element may be controlled by masking or protecting these areas from the action of the etchant solution.
The etchant solution which is mainly used in conventional chemical milling or metal etching is a strong acid or a strong base which dissolves in a controlled manner all surfaces it comes into contact with.
However, a particular problem occurs in relation to etching of aluminium extrusions when such extrusions have a specialised, awkward or other shape which may render the extrusion buoyant in an etching solution. An example of such shape is when the external surface area is substantially in excess of the internal surface area and this may be exemplified by a heat exchanger element having an elongate hollow body and a plurality of outwardly extending fins as described above. The problem is the difficulty of maintaining the heat exchanger element submerged within the etchant solution during the etching process. In this regard, such a heat exchanger element as described above may initially sink to the bottom of an etchant vessel but as milling continues, a chemical reaction occurs at the aluminium surface of the extrusion whereby aluminium will be dissolved. Subsequently, the extrusion will have a tendency to float and thus become a buoyant body and therefore rise upwardly in the etchant solution to the surface. This problem creates considerable difficulties in regard to providing a continuous etching process so as to etch the aluminium extrusion to the required extent.